This invention relates generally to a corona generating device, and more particularly concerns a discorotron.
In a typical electrophotographic printing process, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is exposed to a light image of an original document being reproduced.
Exposure of the charged photoconductive member selectively dissipates the charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet.
The toner particles are heated to permanently affix the powder image to the copy sheet.
In printing machines such as those described above, corona devices perform a variety of other functions in the printing process. For example, corona devices aid the transfer of the developed toner image from a photoconductive member to a transfer member. Likewise, corona devices aid the conditioning of the photoconductive member prior to, during, and after deposition of developer material thereon to improve the quality of the electrophotographic copy produced thereby. Both direct current (DC) and alternating current (AC) type corona devices are used to perform these functions.
One form of a corona charging device comprises a corona electrode in the form of an elongated wire connected by way of an insulated cable to a high voltage AC/DC power supply. The corona wire is partially surrounded by a conductive shield. The photoconductive member is spaced from the corona wire on the side opposite the shield. An AC voltage may be applied to the corona wire and at the same time, a DC bias voltage is applied to the shield to regulate ion flow from the corona wire to the photoconductive member being charged.
Another form of a corona charging device is pin corotrons and scorotrons. The pin corotron comprises an array of pins integrally formed from a sheet metal member that is connected by a high voltage cable to a high power supply. The sheet metal member is supported between insulated end blocks and mounted within a conductive shield. The photoconductive member to be charged is spaced from the sheet metal member on the opposite side of the shield. The scorotron is similar to the pin corotron, but is additionally provided with a screen or control grid disposed between a coronode and the photoconductive member. The screen is held at a lower potential approximating the charge level to be placed on the photoconductive member. The scoroton provides for more uniform charging and prevents over charging.
Still other forms of corona charging devices include a dicoroton. The dicoroton comprises a coronode having a conductive wire that is coated with an electrically insulating material. When AC power is applied to the coronode by way of an insulated cable, substantially no net DC current flows in the wire due to the thickness of the insulating material. Thus, when the conductive shield forming a part of dicorotron and the photoconductive member passing thereunder under at the same potential, no current flows to the photoconductive member or the conductive shield. However, when the shield and photoconductive member are at different potentials, for example, when there is a copy sheet attached to the photoconductive member to which toner images have been electrostatically transferred thereto, an electrostatic field is established between the shield and the photoconductive member which causes current to flow from the shield to ground.
A high speed color machine capable of producing 100 or more images per minute, such as the iGen3(trademark) manufactured by Xerox, requires a charging device capable of delivering uniform charging performance during high speed imaging. Further, there is needed a charging device which is insensitive to toner contamination due to image on image development at high speeds.
There is provided a high speed color printer wherein color images are produced by superposing a developed image atop another developed image, the improvement comprising a charging system for charging a surface, comprising: a coronode, said coronode being a glass coated wire; a housing with said coronode positioned therein; a grid interposed between the surface and said coronode; a shield, interposed between the housing and said coronode; a power supply for biasing said grid and shield; and an AC power supply for energizing said coronode.